Currently, a decrease in CO2 emission is an urgent problem in order to prevent global warming caused by an increase in carbon dioxide (hereinafter, referred to as CO2) concentration in the atmosphere. As measures for decreasing CO2 emission, a decrease in the consumption of fossil fuels through energy saving, measures to be taken in the use of fossil fuels such as the recovery and isolation of CO2 in combustion exhaust gas, and utilization of natural energy such as solar cells and wind power generation, and the like have been promoted. In addition to these, electricity generation by biomass fuels has attracted attention as a method not leading to an increase in CO2. It has started to be adopted in the form of mono-fuel combustion of biomass or in the form of multi-fuel combustion of biomass and fossil fuel particularly in the areas centering on the regions Europe.
By the way, combustion exhaust gases contain nitrogen oxides, and catalysts are used to remove those nitrogen oxides. As a conventional example using such a catalyst, a method is known in which, in the presence of ammonia, exhaust gas is made into contact with a catalyst containing titanium oxide, active ingredients for reaction of removing nitrogen oxides, and metal sulfate having crystallization water (for example, see Patent Literature 1). Also, a method is known in which sulfate ions are adsorbed onto titanium oxide in order to enhance the activation of denitrification catalyst (for example, see Patent Literature 2).
Also, combustion gases of biomass fuel contain a deliquescent potassium carbonate, and this deliquesces to become a liquid state at a low temperature, and then, permeates pores in a denitrification catalyst, resulting in a modification of active sites in the catalyst (for example, see Patent Literature 3).